High performance optical communications systems require transmitters capable of modulating both the amplitude and phase of an optical signal having a high data rate with multi-level precision. In addition, optical modulation formats used for such high performance systems must provide high spectral efficiency, data-rate agility and high receiver sensitivity.
A 16-state modulation format based upon differential quadrature-phase-shift keying (DQPSK) and a 4-level pulse amplitude modulation format (PAM4) has been recently demonstrated. (See, e.g., K. Sekine et al., Proposal And Demonstration Of 10-G symbol/sec, 16-ary (40 Gb/s) Optical Modulation/Demodulation Scheme,” which appeared in Proceedings of European Conference on Optical Communication (ECOC'04), paper We3.4.5, 2004). As is known, the DQPSK-PAM4 format encodes data at 4 bits/symbol and is typically realized by driving a single Mach-Zehnder Modulator (MZM) with a 4-level electronic signal, which in turn is produced by power-combing two signal tributaries of different amplitudes. Unfortunately, this DQPSK-PAM4 scheme exhibits a number of infirmities.
More specifically, and in addition to being overly complex, this scheme results in a less-than-optimal amplitude spacing for the PAM4 modulation since the transfer function of the MZM is pre-determined and cannot be used to produce an optimal spacing (or constellation). Additionally, to approach the optimal spacing, the saturation regime of the MZM transfer function cannot be reached, thereby resulting in additional optical loss and a larger inter-symbol interference (ISI) with a limited modulator bandwidth. Finally, the generation of the multi-level RF signal causes higher ISI in the RF domain and the amplification of the multi-level RF signal requires an amplifier having a large bandwidth and high linearity, which is much harder to implement than a saturating amplifier that is commonly used for binary modulation.